WEAR WARNING ARRANGEMENT ON A SLAVE CYLINDER OPERATED DISC CLUTCH

Abstract: On a slave cylinder operated clutch the slave cylinder (10) is provided with an arrangement for warning when clutch linings are worn. The arrangement incorporates a valve (21) on the inlet line (9) to the slave cylinder. The valve has a free-flowing position and a constricting position. When linings are worn, the valve (21) is caused to assume its constricting position under the influence of a sensor detecting the position of the working piston (13) of the slave cylinder. Application of the constriction makes the clutch pedal become harder to operate, which gives the driver an indication and warning that linings are worn and need changing.

Description: Wear warning arrangement on a slave cylinder operated disc clutch The present invention relates to a wear warning arrangement on a slave cylinder operated disc clutch in accordance with what is indicated in the preamble to the attached patent claim 1. State of the art Disc clutches are subject to continual wear and in order to make it easier to replace the discs at the correct time it is generally desirable to be able to monitor the wear without having to dismantle the whole clutch. In clutches which are operated via a lever, the latter is mechanically connected to the clutch disc plate in a manner which makes direct measurement of wear relatively easy by measuring the position of rest of the lever. In a clutch which is operated by means of a slave cylinder, the whole transfer of movement between the operating device (usually a pedal) and the clutch takes place hydraulically, so there is no lever or other mechanical feedback which would be readily accessible and usable for detecting and indicating disc wear. Instead, it becomes necessary to arrange a sensor directly on the slave cylinder. One way of doing this is described in patent application DE 41 20 643 and its counterpart GB 2 256 907. These describe an embodiment in which wear indication is provided by electrical contacts arranged on the slave cylinder which close when the slave cylinder piston moves beyond a certain limit corresponding to wear, thereby creating the possibility of wear being indicated electrically, e. g. via a warning lamp. In alternative embodiments the piston movement may be detected continuously by means of resistances or capacitances whose values are altered by movement of the slave cylinder piston and are hence also altered by disc wear. These latter variants may also be used for controlling automated clutch systems in which it is for example necessary to be able to control with a certain accuracy the engagement process during clutch operation. A disadvantage of this solution, however, is that it imposes relatively demanding requirements on the components involved. The components of the slave cylinder are subject to both mechanical and thermal stresses. Further components are also required for converting signals from sensors into a form perceptible to the driver. All this leads to solutions which are relatively expensive, at least if their sole object is to indicate lining wear. The aforesaid patent applications also give no indication as to how different slave cylinders may be individually adapted to different clutch discs. This solution therefore supposes that all the components have nominal dimensions, something which rarely happens in practice. This means that it is instead necessary to design the solution taking into account the tolerances which may normally occur and that indications become correspondingly unreliable. Another way of indicating disc wear is described in patent application SE 92033 15-8. In this case the indication that disc wear has exceeded a certain limit is that the driver finds that substantially more effort is required to operate the clutch pedal. This indication is created by change-over of a valve incorporated in a pipe for supplying pressure medium to a servo cylinder under the influence of a piston in the servo cylinder. This solution is intended, however, for the more conventional type of clutch which is operated under the action of a lever and which also incorporates a servo cylinder in which the position of the piston depends on the wear of the discs. This solution is therefore not applicable to a slave cylinder operated clutch. Objects of the invention The present invention has the object of providing an arrangement for indicating clutch disc wear which may be used on a slave cylinder operated clutch. The invention therefore has the object of giving the driver an indication that the discs have worn so much as to require their replacement as soon as possible. This indication is to be primarily given by the driver finding that significantly more effort than normal is required to operate the clutch pedal. Further objects of the invention are that it be easy and inexpensive to manufacture and have no appreciable influence on the design in other respects of the slave cylinder. The invention is moreover intended to have no appreciable influence on the normal functioning of the slave cylinder when no indication is being given, and it also has to be possible for the slave cylinder to be dimensioned for optimum normal functioning. Brief description of the invention According to the invention, the objects aimed at are achieved by the arrangement being designed in accordance with what is indicated in the characterising part of patent claim 1. Arranging a valve on the inlet line to the slave cylinder enables the inflow of hydraulic oil to take place entirely unhindered during normal operation and to take place via an apparent constriction when there is indication of worn linings. This means that the indication can be given both effectively and clearly. A further advantage of the invention is that even if the clutch pedal feels more resistant when linings are worn it nevertheless remains possible to operate the clutch and continue driving the vehicle normally in other respects. The invention means that in normal operation, before linings are so worn as to need changing, the slave cylinder may be optimised and dimensioned for a quick and easily operated clutch. The attached subclaims indicate advantageous embodiments of the invention. The advantages of those embodiments are indicated in more detail by the description below which exemplifies an advantageous embodiment of the invention. List of drawings The description refers to the attached drawings of an arrangement for operating a clutch in a transmission. They depict the following: Fig. 1 a clutch pedal forming part of the arrangement, Fig. 2 a slave cylinder forming part of the arrangement, and a wear sensor arranged on the slave cylinder. The clutch is depicted in a disengaged position and the wear sensor in a position for new linings, and Fig. 3 shows the same slave cylinder as Fig. 2, but with the clutch in an engaged position and the wear sensor in a position for worn linings. Description of an embodiment In this embodiment the invention is described as applied to a vehicle. The invention may also be applied, however, to other types of transmissions. The vehicle driver's compartment is provided with a clutch pedal 1 for operating a disc-type clutch which forms part of the vehicle's transmission between a drive engine and a gearbox. The clutch pedal 1 is bearingly fastened to a bracket on a cowl panel 4 to which a master cylinder 5 is also fastened. The vehicle clutch s only shown schematically in the drawings in the form of a partly depicted conventional diaphragm spring 6 which forms part of the clutch and which is operated by a slave cylinder 10 arranged concentrically about the clutch centreline 8. The slave cylinder 10 is connected to the clutch pedal 1 via a hydraulic line 9 in order to transmit pedal movements to the slave cylinder 10. The slave cylinder 10 incorporates a housing 11 with an inlet connection (not depicted) for supplying hydraulic oil from the master cylinder 5. The slave cylinder 10 incorporates conventionally a cylinder 12 containing a working piston, hereinafter called the operating piston 13, which together with the cylinder 12 delineates the working space 14 of the slave cylinder. Operating movements result in the working space 14 being pressurised by hydraulic oil which acts on the piston with an inserting movement relative to the cylinder 12, which movement is imparted via a disengagement bearing 2 and a sleeve 3 to the fan 6 and thereby operates the clutch in a conventional manner. The clutch is thus of the so-called pulling type, meaning that the clutch during disengagement is acted on by a pulling force. (In alternative embodiments of the invention the clutch may be of a pushing type). For drive power transfer between the engine and the gearbox, the clutch returns to an engaged position under the action of the spring force exerted on it by the fan 6. Since clutches of this type of well-known per se and further information on them is not required for understanding the present invention, no more detailed description of them will be given here. The slave cylinder 10 is provided with an arrangement according to the invention for detecting and giving the driver an indication and warning that clutch disc lingings are worn and need renewing. The invention incorporates a valve 21 arranged on the hydraulic line 9 between the master cylinder 5 and the slave cylinder 10. The design of this valve is depicted in more detail in Figures 2-3. The valve 21 is depicted as a separate unit connected to the slave cylinder 10 but may in alternative embodiments be more or less integrated with the slave cylinder 10. The valve 21 incorporates a valve housing 22 with a cylinder 23 and, contained in the latter, a piston 24. The valve 21 has an inlet 25 connected to the master cylinder 5 and an outlet connected to the inlet connection of the slave cylinder 10. The valve 21 thus forms a connecting link between the master cylinder 5 and the slave cylinder 10. The cylinder 23 takes the form of a through hole which at a shoulder 26 is divided into a lefthand section with a smaller diameter and a righthand section with a larger diameter. The cylinder is sealed at its righthand end by a blanking plate 50 clamped between two locking rings in the cylinder. The valve piston 24 is correspondingly provided with a righthand portion with a larger diameter and a lefthand portion with smaller diameter, which portions are accommodated respectively in the righthand and lefthand sections of the cylinder 23. The piston 24 thus also incorporates a shoulder which divides its lefthand portion from its righthand portion. The shell surface of the valve piston 24 is provided with a number of annular grooves. The lefthand portion of the piston 24 incorporates two O-rings 32 arranged in grooves made for this purpose at the respective ends of the lefthand portion, while the righthand portion of the piston incorporates only one O-ring in a groove made for this purpose. Between the two lefthand O-ring grooves the piston 24 is provided with a first groove 33 and a second groove 34 which merge directly into one another. The first groove 33 is provided with a diameter which is substantially smaller than the diameter of the lefthand portion of the piston 24, while the second groove 34 has a diameter which is only insignificantly smaller than the diameter of the lefthand portion of the piston 24. In an alternative embodiment and with suitably dimensioned radial clearance between the piston 24 and the cylinder 23 the second groove 34 may be omitted and be represented instead by the shell surface of the piston 24 in this region. In Figures 2 and 3 as depicted the difference in diameter between the second groove 34 and the diameter of the piston 24 is so small as not to be visible in these drawings. In the region between the two O-rings 32 of the lefthand portion, there debouch into the cylinder 23, substantially at the same axial position relative to the cylinder 23, not only a duct 35 connected to the inlet 25 of the valve 21 but also a duct (not depicted) connected to the outlet of the valve 21. In the position of rest in which the valve 21 is depicted in Figure 2, these two ducts 35 both debouch centrally relative to the first groove 33, which means that hydraulic oil from the inlet 25 of the valve can pass substantially unhindered to the outlet of the valve via the annular first groove 33. When the valve 21 and the piston 24 assume an activated position, however, the second groove 34 in the piston 24 will assume a position central to the respective orifices of the ducts 35. The second groove 34 will thus act as a constriction which substantially prevents the passage of hydraulic oil, although not more than an certain amount of hydraulic oil can still pass between the master cylinder 5 and the slave cylinder 10. The piston 24 is further provided with a central axial through hole which at both ends of the piston merges into the cylindrical space, which space with reference to Figures 2-3 is termed the lefthand space and the righthand space. Through the hole in the piston 24 extends a tubular sleeve 51 which partly arranged for movement in the piston. The lefthand end of the sleeve 51 is provided with a flange against which abuts one end of a coil spring 53 acting as a compression spring, the other end of which abuts against the inner end of the lefthand space of the piston 24. The spring 53 thus acts upon the sleeve 51 with a force which urges the sleeve to the left and outwards relative to the piston. Inserted in the lefthand end of the sleeve 51 is a resilient tube sleeve 52 which partly extends axially outside the sleeve 51 and there abuts, under action of the spring 53, against a cover 42 which in its turn is firmly connected to the operating piston 13 of the slave cylinder. This means that the sleeve 51 is at least indirectly connected to the operating piston 13 and has the latter's movements imparted to it when the clutch is operated. The inner righthand end of the sleeve 51 extends, as may be seen in Fig. 2, through the righthand cylinder-shaped space in the piston 24 and also into the righthand space of the cylinder 23. Secured firmly to the righthand end of the piston 24 is a sleeve 54, advantageously made of sheetmetal material, which is therefore hereinafter called the sheetmetal sleeve. Between the sheetmetal sleeve 54 and the blanking plate 50 is arranged a coil spring 55 acting as a compression spring, which via the sheetmetal sleeve 54 presses the piston 24 to the left. Movement of the piston 24 to the left is limited, however, by the fact that its righthand portion is too large to be able to enter the lefthand section of the cylinder 23. In the position depicted in Figure 2, the shoulder between the righthand and lefthand portions of the piston 24 abuts against the shoulder 26 situated in the cylinder 23. The axial extent of the sheetmetal sleeve 54 is somewhat shorter than the righthand space of the piston 24. Between the sheetmetal sleeve 54 and the inner end of the space is formed a small space accommodating an annular washer 26 which is positioned radially by the sleeve 51 passing through it. However, the washer 56 is not clamped axially between the sheetmetal sleeve 54 and the inner end of the recess. The outside diameter of the washer 56 is smaller than the diameter of the righthand space of the piston 24, in which righthand space it is also contained. More information on the object of the washer 56 appears in the ensuing description. At the time of fitting the valve 21, the resilient tube sleeve 52 is placed outermost in the sleeve 51. Thereafter, as the working space 14 of the slave cylinder is pressurised, the cover 42 presses the tube sleeve 52 further into the sleeve 51, which abuts at its other end against the blanking plate 50. Thus the total length of the sleeve 51 and the tube sleeve 52 is adapted individually to the position of the particular slave cylinder and the position of the latter's working piston via the cover 42. At the same time the sleeve 51 has thus been brought to a defined position relative to the piston 24 and the cylinder 23. The position of the tube sleeve 52 relative to the sleeve 51 will thus be individually adapted to the particular slave cylinder. The force with which the tube sleeve 52 is pressed into the sleeve 51 is relatively great. For the further movements described below, the tube sleeve 52 and the sleeve 51 may be regarded as entirely firmly connected to one another by the frictional lock which occurs between them. The function of the arrangement described is as follows. During normal operation, when the disc linings are not worn beyond a certain limit, the slave cylinder 10 will conventionally, when the clutch is operated, effect clutch movements in response to the driver's operation of the clutch pedal 1. At the time of a clutch disengaging movement, the working space 14 of the slave cylinder 10 is filled with oil from the master cylinder 5 and pushes the operating piston 13 into the cylinder 12, i. e. to the right in the drawings. Through the action of the spring 53, the movements of the slave cylinder piston 13 will cause the sleeve 51 to be pressed to abut with the tube sleeve 52 against the cover 42 connected to slave cylinder piston. The sleeve 51 will thus move alternately inwards and outwards relative to the piston 24. During these movements the washer 56 is positioned in the radial direction of the sleeve 51. During a return movement after a clutch disengaging movement, the sleeve 51 will move outwards relative to the cylinder 23 and relative to the piston 24, i. e. to the left in Figures 2 and 3. The hydraulic oil which passes to the slave cylinder 10 can pass substantially unhindered via the valve 21 and the annular first groove 33 of the piston 24, thereby causing the driver to find that the pedal 1 can be operated with normal resistance. As the brake linings become worn, the position of rest of the slave cylinder operating piston 13, and hence also the sleeve 51, will move ever further outwards relative to the cylinder 23. Figure 3 shows the clutch in an engaged position but with linings worn beyond a predetermined limit. The valve 21 is depicted in a position in which the wear indication has been triggered and the piston 24 has moved to the right as compared with the position depicted in Figure 2. This has happened in the following manner. At the stage of return movement after a clutch disengagement, the sleeve 51 in its movement to the left has been pushed so far outwards relative to the piston 24 that the inner righthand end of the sleeve 51 has moved past the washer 56 contained in the righthand space. This has caused the washer 56 to be no longer positioned by the sleeve 51, with the result that it has fallen down under the force of gravity and is abutting instead against the wall of the righthand recess of the piston 24. This has meant, at the time of a subsequent clutch disengagement, that the inner end of the sleeve 51 has not been able to pass through the washer 56. Instead, the inner end of the sleeve 51, by abutting against the washer 56, has imparted to the sheetmetal sleeve 54, and to the piston 24 firmly connected to the latter, the movement of the sleeve 51 to the right. During that movement of the piston to the right, the middlemost of the O-rings 32 has moved past the shoulder 26 of the cylinder. During clutch operations, the annular space formed by the first groove 33 will be pressurised. As the sealing effect of the middle O-ring has now ceased, the shoulder between the lefthand and righthand portions of the piston 24 will be subjected to the action of pressurised hydraulic oil. The piston 24 is thus no longer balanced by pressure forces from the hydraulic oil. Pressurisation in connection with clutch operation will therefore cause the piston 24 to move further to the right, relative to the position depicted in Figure 3, with the second groove 34 of the piston assuming a position central to the inlet and outlet of the valve. In this activated position, the valve 21 thus acts as a constricting valve. Hydraulic oil which has to pass through the valve 21 will now be forced to pass the constriction formed by the second annular groove 34 of the piston. This results in the driver finding that movement of the clutch pedal 1 requires substantially more effort than previously, which is a direct indication to him that linings are now so worn as to need renewing. Even if the driver finds that movement of pedal 1 requires greater effort, the clutch still remains fully operable and the vehicle can still be driven normally in other respects. When the clutch returns to an engaged position and the pressurisation of the annular first groove 33 ceases, the piston returns, under the action of the spring 55, to the position depicted in Figure 3. Further movement of the piston 24 to the left is prevented by the middle 0-ring 32 abutting against the shoulder 26 of the cylinder. The force exerted by the spring 55 is in fact insufficient to compress the O-ring sufficiently to enable the piston 24 to move further into the cylinder 23. At the time of subsequent clutch operations, the piston 24 will, in the same manner as described above, move to the right again in connection with pressurisation of the valve 21, through the piston being subjected to a greater force to the right than to the left. Lining renewal restores the valve 21 to the original position in which the washer 56 is once more positioned radially by the sleeve 51. Thereafter the valve 21 is activated anew for further indication when the new linings become worn. The invention may, within the framework of the patent claims set out below, be modified and implemented in ways other than described above. Making the clutch pedal 1 act directly, as previously described, on a master cylinder 5 so as to direct hydraulic oil to the slave cylinder 10 is a solution suited to passenger car use. In the case of heavier vehicles such a truck or a bus, it may instead be more advantageous to use some form of servo device or booster for facilitating clutch operation. In such cases conventional servo devices based on pneumatics and/or hydraulics may be used for providing the servo effect. For operating the slave cylinder 10 and for possibilities of being able to use the invention, how the hydraulic oil pressure imparted to the slave cylinder 10 is generated is unimportant, what is important is that the driver's operating movement is somehow converted to a hydraulic oil pressure which can be imparted to the slave cylinder 10. The presence of servo devices does of course mean that the forces with which the clutch is operated are of a different magnitude than when servo devices are not used, but the relative difference in pedal force which the driver detects between new and worn linings remains palpable, given suitable dimensioning, even when servo devices are present. Instead of using a slave cylinder operated by means of hydraulic oil, it is of course possible to use a slave cylinder for other pressure media. In the embodiment described above, Figure 3 shows the valve piston (24) in a triggered position in which the flow through the valve is only partly constricted, and only on pressurisation in connection with clutch operation is the piston brought to a more constricting position. As this triggered position does in practice cause the same results as regards indications to the driver as if the piston had been brought directly to the more constricting position, the constricting position in the attached patent claims should also be regarded as including such intermediate positions, despite the fact that they may in practice constitute different positions. In alternative embodiments, however, this triggered position may be the same as the more constricting position. The embodiment described refers to a vehicle in which the clutch is operated by means of a clutch pedal. When the invention is used in other (non-vehicle) transmissions, the clutch may instead be operable via other operating devices instead of a pedal.

Claims: Patent claims 1. Wear warning arrangement on a slave cylinder operated clutch, preferably forming part of a vehicle drive unit, incorporating a slave cylinder (10) which is arranged concentrically with the clutch and has a pressure medium operated working piston (13) arranged in a working space (14) in the slave cylinder which, when the clutch is operated by an operator, is supplied with pressure medium via an inlet line (9) on which a sensor is arranged to give, by detecting the position of the working piston (13), an indication when clutch linings are worn, c h a r a c t e r i s e d in that a valve (21) is arranged on the inlet line (9) to the working space (14) of the slave cylinder, that the valve (21) in a first position substantially without hindrance can conduct pressure medium to the working space (14), that the valve (21) in a second position is arranged to apply a constriction (34) in the inlet line (9), and that the valve (21) is controlled under the influence of the sensor so that it switches, directly or indirectly, from the first position to its second position when the sensor indicates that the degree of lining wear exceeds a predetermined value, in order thereby to apply the constriction (34) and delay the pressure medium supply during clutch operation, thereby giving the operator a wear indication by increased resistance to clutch operation. 2. Arrangement according to patent claim 1, c h a r a c t e r i s e d in that the valve (21) incorporates a valve housing (22) provided with a cylinder (23) containing a valve piston (24), that the valve piston (24) in cooperation with the cylinder (23) delineates a sealed space, and that the sealed space forms part of the inlet line (9) to the working space (14) of the slave cylinder. 3. Arrangement according to patent claim 2, c h a r a c t e r i s e d in that the valve piston (24) is provided with an axial through hole which accommodates a sleeve (51) running through it which on one side of the valve piston (24) is at least indirectly connected to the slave cylinder working piston (13) and which on the other side of the valve piston (24) is provided with devices (54,56) by means of which, when clutch linings are worn, clutch movements may result in axial movement being imparted to the valve piston (24) so that it assumes its second position at which the constriction (34) is applied so as to delay the pressure medium supply to the working space (14) of the slave cylinder. 4. Arrangement according to patent claim 3, c h a r a c t e r i s e d in that when linings are worn the valve piston (24) is arranged to be acted upon, at least indirectly, via the sleeve (51) and devices cooperating with the latter, by the slave cylinder working piston (13) during clutch operation, until it assumes the second position. 5. Arrangement according to patent claim 1, c h a r a c t e r i s e d in that the valve piston (24) is provided with a shoulder which separates a portion of the valve piston (24) with a smaller diameter and a portion of the valve piston (24) with a larger diameter, that the cylinder (23) is provided with a corresponding shoulder (26) which divides the cylinder (23) into a section with a smaller diameter and a section with a larger diameter, and that the valve piston (24) is arranged so that in its second position it causes the shoulder of the valve piston (24), to be subjected during clutch operation to pressure medium which causes the valve piston (24) to assume the position which constricts flow through the valve (21). 6. Arrangement according to patent claim 5, c h a r a c t e r i s e d in that the valve piston (24) is provided with a first groove (33) and a second groove (34) which respectively define a passage and substantially hinder a passage with a constriction through the valve (21). 7. Arrangement according to patent claim 4, c h a r a c t e r i s e d in that the devices which cooperate with the sleeve (51) and by means of which, when clutch linings are worn, clutch movements may impart axial movement to the valve piston (24) so that the latter assumes its second position, consist of a washer (56) and a sleeve (54) which is firmly connected to the valve piston (24). 8. Arrangement according to patent claim 7, c h a r a c t e r i s e d in that the sleeve (51) in the region which is at least indirectly connected to the slave cylinder working piston (13) incorporates a spacing device (52) by means of which a length adaptation of the sleeve (51) can be effected relative to a particular slave cylinder (10). 9. Arrangement according to patent claim 7, c h a r a c t e r i s e d in that the washer (56) in a position which does not constrict the valve piston (24) is positioned radially by the sleeve (51) and that the sleeve (51) is designed so that when the degree of lining wear exceeds the predetermined value the radial positioning of the washer (56) is released at the time of a clutch engaging movement.

Inventors: NORDKVIST JOHAN, NORDKVIST, JOHAN
Applicants: SCANIA CV AB, SCANIA CV AKTIEBOLAG
Application year: 1994
Main classification: F16D25
IPC codes: F16D25